Field of the Invention
The present invention relates to a wireless relay device, a processing apparatus, a wireless communication system, and a wireless communication method.
Priority is claimed on Japanese Patent Application No. 2015-162455, filed on Aug. 20, 2015, the contents of which are incorporated herein by reference.
Description of Related Art
A distributed control system (DSC) has been built in plants, factories, or the like, thereby a high level of automated operation has been realized. In the distributed control system, on-site devices (measures and actuators), which are referred to as field devices, are connected via a communication means to controllers that controls the field devices. The communication system that forms the base of such a distributed control system had been almost always one that communicates by cable. A wireless communication system that communicates wirelessly in conformance with an industrial wireless communication standard such as ISA100.11a or WirelessHART (registered trademark) has been realized in recent years.
In such a wireless communication system, various data with regard to the process control is transmitted and received via a wireless network. For example, in the wireless communication system in conformance with the above-described ISA100.11a, the wireless network is formed by field devices capable of performing wireless communication (wireless field devices), wireless routers, and backbone routers, packets including sensor data (Publish packets) are transmitted from the wireless field devices, via the wireless routers and backbone routers, to a host system.
Japanese Unexamined Patent Application, First Publication No. 2014-86852 (hereinafter, referred to as Patent Document 1) discloses an example of wireless communication system in conformance with the above-described ISA100.11a, WirelessHART (registered trademark), or the like. Specifically, Patent Document 1 discloses an invention capable of making wireless devices greater than before participate in a wireless network, by making wireless devices participate in the wireless network, which transmit a join request to the wireless network, and making the wireless devices be separated from the wireless network after the wireless communication via the wireless network is finished.
The number of field devices provided in plants or the like increases or decreases based on, generally, the scale of plants or the like, and there are some cases in which, in a large plant or the like, the number of field devices exceeds ten-thousands. Therefore, there is a possibility that, in a large plant or the like, a large wireless network (for example, a wireless network to which wireless field devices greater than ten-thousands, and the like are connected) is built in the future. However, at this time, it is not easy to build a large wireless network due to, mainly, the following two reasons.
(1) Increase in Communication Load
Since the number of field devices is large in a large wireless network, the communication load of wireless relay devices provided in the communication path is increased. For example, in the above-described wireless communication system in conformance with ISA100.11a, when the number of wireless field devices under wireless routers is increased, the communication load of the wireless routers is increased. When the number of wireless routers under backbone routers is increased, the communication load of the backbone routers is increased. If the number of devices is restricted (wireless field devices under wireless routers, or wireless routers under backbone routers) to prevent such an increase of communication load, the building of a large wireless network becomes difficult.
(2) Lack of Wireless Band Resources
Since the number of packets transmitted and received via a wireless network is increased in proportion to the increase of the number of devices connected to the wireless network, the wireless band required for the transmitting and receiving of packets is increased. Since the allocatable wireless band is finite in a wireless communication system, the wireless band resources become scarce as the number of devices connected to the wireless network is increased. Such a lack of wireless band resources restricts the number of devices connectable to the wireless network, and thereby the building of a large wireless network becomes difficult.